Timing adjustment for distributed network architecture

ABSTRACT

In some embodiments, a first computing device detects a loss of a connection to a first source of timing information that the first computing device and a second computing device use to maintain synchronization with a first clock and a second clock. The first computing device receives a second source of timing information from the second computing device. The second source of timing information is also being transmitted to a third computing device. The first computing device uses the second source of timing information to determine a first timestamp and determines a second timestamp from the first clock. The first computing device uses the first timestamp and the second timestamp to adjust a rate of the first clock where the first clock is used to transmit the second source of timing information from the second computing device to the third computing device.

BACKGROUND

In a network implementation, such as a data over cable service interfacespecification (DOCSIS), a physical (PHY) device can be located in theheadend that converts packets on a digital interface, such as anEthernet interface, to analog signals, such as radio frequency (RF)signals, on a hybrid fiber coaxial (HFC) network. The physical devicesends the RF signals to modems located at a subscriber's premises.However, other implementations have moved the physical device to alocation closer to the subscriber's premises, such as in a node locatedin the neighborhood where the subscribers are located. The relocatedphysical device is referred to as a remote physical device (RPD).

Relocation of the physical device to a remote location from the headendrequires that a headend device, such as a converged cable accessplatform (CCAP) core, maintain timing synchronization with the RPD. Inone example, a protocol, such as the 1588 protocol, is used tosynchronize the timing by passing messages between a 1588 timing serverand the headend device and also the 1588 timing server and the RPD. Themessages compare timestamps from the 1588 timing server with a localcopy of a timestamp in both the headend and the remote physical device.The differences between the timestamps of the 1588 timing server andeither the headend device or the RPD are used to adjust the respectiveclocks of the headend device and RPD to make sure the local clocks trackthe timing server's clock. This maintains the timing synchronizationbetween the headend device and the RPD. However, if the 1588 timingserver was to go offline for any significant duration, the clock of theRPD might drift. If the clock of the RPD drifts, modems coupled to theRPD may go offline due to the mismatch in timing between the RPD clockand the headend clock.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a simplified system of a method for maintaining timingsynchronization according to some embodiments.

FIG. 2 depicts a simplified flowchart of a method for adjusting a RPDclock according to some embodiments.

FIG. 3 depicts a simplified flowchart of a method for adjusting the RPDclock using minislot values according to some embodiments.

FIG. 4 depicts a more detailed example of a remote physical deviceaccording to some embodiments.

FIG. 5 depicts a simplified flowchart of a method for adjusting a CCAPclock according to some embodiments.

FIG. 6 depicts a simplified flowchart of a method for using the minislotvalues to adjust the CCAP clock according to some embodiments.

FIG. 7 depicts a more detailed example of a headend according to someembodiments.

FIG. 8 illustrates an example of a special purpose computer systemsconfigured with components of the headend and/or the remote physicaldevice according to one embodiment.

DETAILED DESCRIPTION

Described herein are techniques for a clock adjustment system. In thefollowing description, for purposes of explanation, numerous examplesand specific details are set forth in order to provide a thoroughunderstanding of particular embodiments. Particular embodiments asdefined by the claims may include some or all of the features in theseexamples alone or in combination with other features described below,and may further include modifications and equivalents of the featuresand concepts described herein.

In a network, a physical device (PHY) in a node may be separated fromthe media access control (MAC) device in a headend. For example, in adistributed converged cable access platform (CCAP), a remote physicaldevice (RPD) is separated from the MAC device (e.g., a CCAP core). Atiming server may maintain a timing synchronization between the CCAPcore and the remote physical device. For example, the timing server mayuse a protocol, such as the 1588 protocol, to maintain synchronizationbetween a CCAP clock in the headend and an RPD clock in the node.

When the timing synchronization is lost, such as in the CCAP core or theremote physical device, some embodiments detect the loss and then canadjust either the remote physical device clock or the CCAP clock tomaintain timing synchronization such that modems downstream from the RPDdo not go offline. The adjustment may use a secondary source of timinginformation different from the timing server to adjust the RPD clock orCCAP clock. For example, the remote physical device may receivebandwidth allocation maps (MAPs) that define minislots in which themodems can transmit upstream. The remote physical device can convert aminislot into a first timestamp. The remote physical device also candetermine a second timestamp (e.g., the current time) from the RPD clockand compare the first and second timestamps. The first timestamprepresents that time that a modem can transmit upstream, which should besometime in the future compared to the second timestamp. The firsttimestamp is in the future because the remote physical device stillneeds to send the MAP to the modem, which then needs to process the MAPbefore the first timestamp is reached. If the modems receive thebandwidth allocation maps late and miss respective minislot times totransmit upstream, eventually the modems will go offline.

The remote physical device uses the difference between the firsttimestamp and the second timestamp to adjust the RPD clock, if needed.For example, before the timing synchronization was lost, the RPDmonitors a time difference between the RPD timestamp and the minislottimestamp over a time period. The difference typically averages a time,such as 5 milliseconds (ms) such that modems can receive the MAP andhave time to process the MAP and transmit upstream during the allocatedminislot. The remote physical device then calculates the differencebetween the RPD timestamp and the minislot timestamp. The remotephysical device inputs the difference into a RPD clock adjustmentengine, which can use the difference to adjust the RPD clock, if needed.In some examples, the remote physical device receives many bandwidthallocation maps every few milliseconds and can continuously calculatethe difference and input the difference into the RPD clock adjustmentengine. As the differences change, the RPD clock adjustment engine canadjust the clock to be faster or slower, if needed. The adjustment keepsthe difference between the RPD timestamp and minislot timestamp aroundthe same as before timing synchronization was lost with the timingserver. By keeping the difference around the same, the modems have timeto receive the bandwidth allocation map, process the bandwidthallocation map, and then transmit upstream when the modems' respectiveminislot is encountered. The adjustment thus keeps the modems from goingoffline because the modems still receive the bandwidth allocation mapswithin a time that allows the modems to transmit upstream duringrespective minislots. The process does not lock both the RPD clock andthe CCAP together to a time; however, the process attempts to keep themodems online until the connection to the timing server is restored.

FIG. 1 depicts a simplified system 100 of a method for maintainingtiming synchronization according to some embodiments. System 100includes a headend 102, a remote physical device 104, and a modem 106.Headend 102 and remote physical device 104 may be separated by anetwork, such as a digital network, (e.g., an Ethernet of an opticalnetwork). For example, remote physical device 104 may be located in anode that is located closer to the premises of a subscriber compared toheadend 102. The premises of the subscriber includes a network device,such as a modem 106 (e.g., a cable modem). Although modem 106 isdescribed, other devices may be used, such as a gateway, customerpremise equipment (CPE) including a set top box, etc. Although thisarchitecture is described, other distributed architectures may be used.Further, the processes described may also be applied to differentarchitectures in which time synchronization needs to be maintainedbetween multiple devices.

In some examples, the MAC and PHY layers are split and the PHY layer ismoved to remote physical device 104 while a CCAP core 112 performs mediaaccess control (MAC) layer processing. CCAP core 112 contains both acable modem termination system (CMTS) core for DOC SIS and an edgequadrature amplitude modulation (EQAM) core for video. The CMTS corecontains a DOCSIS MAC component that uses DOCSIS to process allsignaling functions downstream and upstream for bandwidth scheduling andDOCSIS framing. The EQAM core is associated with video processingfunctions.

In some examples, CCAP core 112 sends media access control (MAC)messages over a digital medium, such as Ethernet or a passive opticalnetwork (PON), to remote physical device 104. The MAC messages arereceived as electrical signals at remote physical device 104 and remotephysical device 104 converts the electrical signals to an analog signal,such as a radio frequency (RF) signal. Remote physical device 104 sendsthe RF signal over an analog medium, such as a coaxial network, to modem106. Modem 106 can then provide the signals to various subscriber enddevices. The subscriber end devices may also send data upstream andmodems 106 may transmit the data from the subscriber end devicesupstream to remote physical device 104. Remote physical device 104 thenconverts the RF signals to digital and sends the digital signals to CCAPcore 112. Additionally, modems 106 may transmit upstream to maintain aconnection to remote physical device 104, which may be referred to asranging.

Remote physical device 104 includes downstream QAM modulators andupstream QAM de-modulators to modulate downstream and upstream traffic.Also, remote physical device 104 converts downstream DOCSIS signalingvideo, and out-of-band signals received from CCAP core over the digitalmedium to analog for transmission over radio frequency or linear optics.Remote physical device 104 converts upstream DOCSIS signals receivedfrom an analog medium to digital for transmission over the digitalmedium and sends those signals to CCAP core 112.

As discussed above, because the remote physical device is locatedseparately from headend 102, remote physical device 104 needs tomaintain timing synchronization with CCAP core 112. System 100 usestiming server 114 to maintain timing synchronization between CCAP core112 and remote physical device 104. Timing server 114 may be a separatedevice from headend 102 and remote physical device 104. In otherexamples, timing server 114 may be situated in headend 102. Also, one ormore timing servers 114 may be used, such as a first timing server 114may be used to synchronize timing with headend 102 and a second timingserver 114 may be used to synchronize timing with remote physical device104. Timing server 114 may use a protocol, such as the 1588 protocol, tosynchronize the timing between headend 102 and remote physical device104. The synchronization locks CCAP clock 116 and RPD clock 108 to atime that is synchronized with timing server 114. That is, each clockmay operate at the same rate.

As discussed above, at some points, the timing synchronization may belost, such as timing server 114 may go offline or there may be somenetwork elements, such as switches, that prevent communication betweentiming server 114 and headend 102 and/or remote physical device 104.When timing synchronization is lost with timing server 114, someembodiments use a process to maintain timing synchronization such thatmodems 106 that are coupled to remote physical device 104 do not gooffline. This may not lock CCAP clock 116 and RPD clock 108 to a time orrate together, but adjusts either CCAP clock 116 and RPD clock 108 tokeep modems 106 online. CCAP clock 116 and RPD clock 108 may be based onan oscillator whose frequency can be adjusted, such as digitally.

Remote physical device 104 or CCAP core 112 may use a second source oftiming information to adjust either CCAP clock 116 or RPD clock 108 suchthat communications in system 100 can continue. In some embodiments, thesecond source of timing is received in communications being sent fromCCAP core 112 for modem 106, such as in a bandwidth allocation map. Thebandwidth allocation maps include minislots that define times in whichmodems 106 can send bursts upstream. Each minislot value in thebandwidth allocation map corresponds to the start of each allocationgrant. In the process, remote physical device 104 sends the bandwidthallocation maps downstream to modems 106. Modems 106 can then read thebandwidth allocation maps and determine when an allocation grant isprovided to it to determine when to send a burst upstream. However, ifmodem 106 receives the bandwidth allocation map for a time in the past,then that modem 106 has missed its time to transmit upstream. Thisscenario might occur if RPD clock 108 is going too fast relative to CCAPclock 116. For example, remote physical device 104 may need to receivebandwidth allocation maps by a certain time, such as 5 milliseconds(ms), before a minislot time in which an upstream burst is scheduled fora first modem 106 in the bandwidth allocation map. If a modem 106receives the bandwidth allocation map after a minislot time for thatmodem 106, then that modem 106 will miss its minislot to send a burstupstream. If multiple upstream bursts are missed, then a modem 106 maygo offline. It is possible that a late bandwidth allocation map maycause modems corresponding to earlier minislots to miss their slots, butmodems corresponding to later minislots make receive the bandwidthallocation map in time to send the bursts upstream.

CCAP core 112 sends bandwidth allocation maps periodically, such asevery few milliseconds, to remote physical device 104. CCAP core 112uses CCAP clock 116 to generate the bandwidth allocation maps. Then,remote physical device 104 uses RPD clock 108 to send the bandwidthallocation maps to modem 106. After losing a connection with timingserver 114, when RPD clock 108 starts to drift, an RPD clock adjustmentengine 110 in remote physical device 104 uses the bandwidth allocationmaps to adjust a rate of RPD clock 108. For example, RPD clockadjustment engine 110 may adjust the rate of RPD clock 108 such that atime difference between a RPD timestamp of RPD clock 108 and theminislot timestamp remains substantially constant, such as substantiallyaround the time difference that existed before timing synchronizationwas lost. It is possible that RPD clock 108 may drift in comparison toCCAP clock 116; however, maintaining the time difference allows remotephysical device 104 to send the bandwidth allocation maps to modems 106in enough time such that modems 106 do not miss minislot times totransmit upstream.

In addition to RPD clock 108 possibly losing timing synchronization,CCAP core 112 may also lose timing synchronization with timing server114. In some examples, a CCAP clock adjustment engine 118 can useupstream signals, such as bursts from modem 106, to maintain timingsynchronization. As with RPD clock adjustment engine 110, CCAP clockadjustment engine 118 is not maintaining a lock with RPD clockadjustment engine 110 when timing synchronization is lost with timingserver 114, but rather adjusting the CCAP timing based on the upstreambursts that are received. For example, modems 106 may send upstreambursts that include the minislot times associated with their upstreambursts. CCAP clock adjustment engine 118 can then use those minislottimes to adjust CCAP clock 116. For example, if the burst arrives laterthan expected, then, maybe that CCAP clock 116 is too slow and CCAPclock adjustment engine 118 adjusts CCAP clock 116 to run faster.

The following will now describe both adjustment processes in moredetail.

RPD Clock Adjustment

FIG. 2 depicts a simplified flowchart 200 of a method for adjusting RPDclock 108 according to some embodiments. At 202, RPD clock adjustmentengine 110 detects that a connection to timing server 114 is offline(e.g., a connection is lost). RPD clock adjustment engine 110 may detectthe loss of the connection based on messages not being received fromtiming server 114 or some delay in receiving messages. For example,remote physical device 104 should receive messages from timing server114 periodically at certain defined intervals. If a number of messagesover a threshold are not received or received late, then RPD clockadjustment engine 110 may determine that the connection with timingserver 114 is offline.

At 204, RPD clock adjustment engine 110 receives a second source oftiming information. For example, CCAP core 112 may generate bandwidthallocation maps that include information that RPD clock adjustmentengine 110 can convert into timing information. As discussed above, theminislot value contains a time slot that defines when a modem 106 cantransmit a burst upstream. Although the minislot value is discussed,other information that defines when communications can occur in system100 may be used. For example, the information received from CCAP core112 may define a specific time when modem 106 can transmit upstreaminstead of a minislot value or when remote physical device 104 cantransmit downstream instead of when modem 106 can transmit upstream.Thus, minislot values do not need to be used. Also, the second source oftiming information may be other timing values that RPD clock adjustmentengine 110 can use, such as any timing values that can be derived fromcommunications that are performed in system 100 while the timingsynchronization is lost. One advantage of using minislot values is thatthe bandwidth allocation maps are typically sent and also sent veryfrequently. Thus, no extra bandwidth is being added to system 100 byusing the already being sent bandwidth allocation maps.

At 206, RPD clock adjustment engine 110 converts the second source oftiming information to a first timestamp. The first timestamp may be afirst time in which a modem 106 can transmit upstream. The conversionfrom the minislot value to the first timestamp may depend upon theconfiguration of system 100. For example, minislots may be separated bya time interval. RPD clock adjustment engine 110 can take the minislotvalue and determine a time value that can be compared to RPD clock 108.For a first configuration, such as an Advanced Time Division MultipleAccess (ATDMA) upstream channel type, a minislot is configured to be apower of 2 multiple of the DOCSIS 6.25 microsecond tick. Thus, minislotsizes would be 6.25 μs, 12.5 μs, 25 μs, 50 μs etc. Therefore, thetimestamp in microseconds is derived by multiplying the minislot countby the number of microseconds per minislot. The minislot count is a 26bit number and laps in about 7 minutes so this is extended to a 64 bitnumber by comparison with the current time. For another configuration,such as a Synchronous Code Division Multiple Access (SCDMA) channeltype, a frame is configured with a number of minislots and a startreference timestamp. Thus, any future timestamp can be calculated basedon the start reference and the number of elapsed minislots since thatreference. In other examples, the bandwidth allocation map may includethe first timestamp and RPD clock adjustment engine 110 does not need toperform the conversion.

At 208, RPD clock adjustment engine 110 compares the first timestamp toa second timestamp of RPD clock 108. The second timestamp may be acurrent time of RPD clock 108. RPD clock adjustment engine 110 maydetermine a difference between the second timestamp and the firsttimestamp using the comparison. This difference may determine the amountof time that modem 106 has to process the bandwidth allocation mapbefore having to transmit the upstream burst. For example, if thedifference from the second timestamp to the first timestamp is 5milliseconds, which is the current time compared to a time in thefuture, then the first modem 106 that is scheduled to transmit in thefirst minislot should have around 5 milliseconds to receive thebandwidth allocation map, process it, and then be able to transmit aburst upstream. The difference may be determined over receiving multiplebandwidth allocation maps while timing synchronization is maintainedwith timing server 114. For example, remote physical device 104 receivesmany bandwidth allocation maps over time and RPD clock adjustment engine110 can determine the standard or average difference between the firsttimestamp and the second timestamp while timing synchronization isactive. Remote physical device 104 may use a certain percentage of pastbandwidth allocation maps, such as the last 10%.

At 210, RPD clock adjustment engine 110 adjusts RPD clock 108, ifneeded. RPD clock adjustment engine 110 then attempts to maintain thatdifference when timing synchronization is lost. That is, the differencemay drift from 5 milliseconds to 4.9 milliseconds to 4.8 milliseconds,etc. Then, RPD clock adjustment engine 110 attempts to adjust the rateof RPD clock 108 to maintain the 5 millisecond difference. For example,RPD clock adjustment engine 110 may slow down the rate of RPD clock 108to increase the difference if the difference is decreasing. Slowing downthe RPD clock 108 reduces the rate at which maps are consumed by RPD 104so the map advance time will increase back up to 5 ms. However, if thedifference is increasing, such as from 5 milliseconds to 5.1milliseconds, to 5.2 milliseconds, RPD clock adjustment engine 110speeds up the rate of RPD clock 108 to counteract the increase in thedifference. Speeding up the RPD clock 108 increases the rate at whichmaps are consumed by RPD 104 so the map the advance time will decreasedown to 5 ms.

FIG. 3 depicts a simplified flowchart 300 of a method for adjusting RPDclock 108 using minislot values according to some embodiments. At 302,RPD clock adjustment engine 110 receives a bandwidth allocation map. At304, RPD clock adjustment engine 110 converts a minislot value in thebandwidth allocation map to a first timestamp. For example, RPD clockadjustment engine 110 may use the first minislot value in the bandwidthallocation map. This minislot value may define the first time in which afirst modem 106 can transmit a burst upstream. At 306, RPD clockadjustment engine 110 computes a difference between the first timestampand the second timestamp.

At 308, RPD clock adjustment engine 110 inputs the difference betweenthe first timestamp and the second timestamp into a control system, suchas a phase locked loop (PLL). The phase locked loop may take inputvalues and generate an output signal that is related to the phase of theinput signal. In some embodiments, a Proportional-Integral (PI)controller may be used. Phase locked loops may be used to keep thedifference that was determined while timing synchronization with timingserver 114 constant when timing synchronization is lost. Although aphase locked loop is described, other control systems may be used toadjust RPD clock 108, if needed. At 310, the control system modifies RPDclock 108, if needed, such as by speeding up or slowing down the clockrate.

In some embodiments, the set point of the PI-controller would be currentclock value+a sufficient advance (e.g., 5 ms) added to it for the mapsto get to the modems 106 on time. The measured value would be theaverage of the timestamps derived from the minislot count in thebandwidth allocation maps. The error would be the difference between theset-point and the measured value. The PI controller would output anadjustment to apply to the oscillator of RPD clock 108 to slow the clockif the sign of the error indicated that it was running too fast andvice-versa.

Remote Physical Device

FIG. 4 depicts a more detailed example of remote physical device 104according to some embodiments. A receiver 402 receives the bandwidthallocation maps. For example, receiver 402 may be a burst receiver thatreceives the maps. In some cases, the maps may be sent as normaldownstream channel data and also sent to receiver 402.

A timestamp generator 404 generates the first timestamp from thebandwidth allocation maps. Additionally, timestamp generator 404 maygenerate the second timestamp based on the current time from RPD clock108. Timestamp generator 404 may input the difference between the firsttimestamp and the second timestamp into RPD clock adjustment engine 110.However, clock adjustment engine 110 may not always use the differenceto adjust RPD clock 108. Rather, prior to losing the connection, RPDclock uses 1588 timer engine 406 to maintain timing with timing server114.

Once the connection is lost, 1588 timer engine 406 may notify clockadjustment engine 110. Then, RPD clock adjustment engine 110 may adjustRPD clock 108 based upon the difference received from timestampgenerator 404. Accordingly, RPD clock 108 is now synchronized usingclock adjustment engine 110 and not 1588 timer engine 406.

Headend 102 Clock Adjustment

In some embodiments, CCAP clock 116 may lose timing synchronization.CCAP clock adjustment engine 118 may then maintain the timingsynchronization using a second source of timing information.

FIG. 5 depicts a simplified flowchart 500 of a method for adjusting CCAPclock 116 according to some embodiments. At 502, CCAP clock adjustmentengine 118 detects that the connection with timing server 114 isoffline. The detection may be based on headend 102 not receivingmessages from timing server 114 or receiving the messages late. CCAPcore 112 may have a more precise clock than RPDs 108 so CCAP clock 116may drift very slowly. However, given enough time, CCAP clock 116 willdrift significantly enough for modems 116 to go offline. In the case ofa virtualized implementation of a CCAP core 112, a precise oscillatormay not be in use so drift would occur more rapidly.

At 504, CCAP clock adjustment engine 118 receives a second source oftiming information sent from remote physical device 104. The bandwidthallocation maps are generated by CCAP core 112 so cannot be used toadjust the timing of CCAP clock 116. However, modems 106 may sendsignals, such as bursts upstream, in the minislots allocated to them inthe bandwidth allocation map. The bursts upstream may include theoriginal minislot values. For example, a modem 106 includes the minislotvalue that is assigned to it in its burst upstream. The time that theburst is sent from the RPD 104 to the CCAP core 112 depends on the burstduration because RPD 104 waits for the burst to be received before it issent to CCAP core 112. This creates a significant variation in thearrival time of bursts relative to the minislot value contained withinthem. One form of filtering is to ignore the timestamps of all burstsexcept a percentage, such as 1%, that have the smallest differencebetween the first and second timestamps. This should correspond to thebursts that were sent to CCAP core 112 the soonest after they werereceived by RPD 104. Although minislot values are described, otherinformation may be included that is a source of timing information. Forexample, timestamps in which the bursts sent upstream may be included.

At 506, CCAP clock adjustment engine 118 converts the second source oftiming information to a first timestamp. For example, CCAP clockadjustment engine 118 can perform the same conversion as described withrespect to remote physical device 104 to convert minislot values totimestamps.

At 508, CCAP clock adjustment engine 118 compares the first timestamp toa second timestamp of the CCAP. For example, CCAP clock adjustmentengine 118 uses the current time from CCAP clock 116 as the secondtimestamp. At 510, CCAP clock adjustment engine 118 adjusts CCAP clock116, if needed. The minislot value is the time allocated to transmit aburst upstream. Headend 102 should receive the burst shortly thereafter.Thus, the current time of CCAP clock 116 should be relatively close tothe first timestamp after filtering out the larger bursts that likelyhad a delayed transmission to CCAP core 112. If the earliest burstsupstream are arriving later than expected at headend 102, then it ispossible that a rate of CCAP clock 116 is too slow. The differencebetween the first timestamp and the second timestamp may be gettinglarger in this case. Accordingly, CCAP clock adjustment engine 118 mayadjust a rate of CCAP clock 116 to run faster. However, if the timedifference is becoming smaller, then CCAP clock adjustment engine 118may adjust a rate of CCAP clock 116 to run slower. As described withrespect to remote physical device 104, there may be a difference betweenthe first timestamp and the second timestamp that may be establishedwhen timing synchronization with timing server 114 was online. However,because the minislot value is closer to the current time at headend 102,the difference at CCAP adjustment engine 118 is different from RPD clockadjustment engine 110. Nevertheless, CCAP adjustment engine 118 attemptsto maintain the difference that was previously determined while timingsynchronization was online. CCAP adjustment engine 118 may also use aPLL to adjust CCAP clock 116 in a similar way as described above. Forexample, the difference between the burst timestamp (which is derivedfrom the minislot count of the start of the upstream burst) and the CCAPclock timestamp is input to the PLL, which adjusts the CCAP clock 116.

FIG. 6 depicts a simplified flowchart 600 of a method for using theminislot values to adjust CCAP clock 116 according to some embodiments.At 602, CCAP clock adjustment engine 118 receives an upstream burst fromremote physical device 104 with a minislot value. At 604, CCAP clockadjustment engine 118 converts the minislot value to a first timestamp.At 606, CCAP clock adjustment engine 118 compares the first timestamp toa second timestamp for the CCAP.

At 608, CCAP clock adjustment engine 118 inputs the difference betweenthe first timestamp and the second timestamp into a control system. Forexample, as discussed above, a phase locked loop or other controlsystems may be used. At 610, CCAP clock adjustment engine 118 adjuststhe CCAP clock, if needed. For example, the phase locked loop mayattempt to keep the difference established while timing server 114 wasonline constant during the time in which timing server 114 is offline.

FIG. 7 depicts a more detailed example of headend 102 according to someembodiments. A receiver 702 receives an upstream burst from remotephysical device 104. As discussed above, the burst may include minislotvalues. Timestamp generator 704 uses the minislot values to generate thefirst timestamp. Also, timestamp generator 704 generates the secondtimestamp based on CCAP clock 116. Timestamp generator 704 may input thedifference between the first timestamp and the second timestamp intoCCAP clock adjustment engine 118. However, CCAP clock adjustment engine118 may not always use the difference to adjust CCAP clock 116. Rather,prior to losing the connection, CCAP clock 116 uses 1588 timer engine708 to maintain timing with timing server 114.

When a lost connection is detected, 1588 timer engine 708 sends anindication to CCAP clock adjustment engine 118. Then, CCAP clockadjustment engine 118 may adjust CCAP clock 116 based upon thedifference received from timestamp generator 704. Accordingly, CCAPclock 116 is now synchronized using CCAP clock adjustment engine 118 andnot 1588 timer engine 708. CCAP core 112 can synchronize itself to RPDclock 108 when CCAP core 112 has lost its own timing source. However, insome instantiations, a large number of RPDs 104 may be handled by asingle CCAP core. Therefore, the CCAP core is synchronizing itself tothe average of all of those RPDs 104.

CONCLUSION

Accordingly, some embodiments use a second source of timing informationto maintain either CCAP clock 116 or RPD clock 108. The timingsynchronization that is maintained does not lock CCAP clock 116 to RPDclock 108. Rather, the clock adjustment attempts to adjust one of theclocks such that modems 106 do not go offline. The adjustment usestiming information that is used in communications in system 100, such asminislot values in a bandwidth allocation map. This leveragesinformation that is already being sent, such as from headend 102 tomodem 106, or vice versa. Although information already being sent isdescribed, it is possible that headend 102 may generate timinginformation that can be sent when the connection is lost with timingserver 114.

At some point, timing server 114 may come back online. This allows CCAPcore 112 and RPD 104 to reestablish timing with timing server 114, suchas both can establish a 1588 lock after the interruption has ceased.Some embodiments adjust RPD clock 108 or CCAP clock 116 back towards thetime of timing server 114 slowly without service interruption becauseeither clock may not have drifted too far during the interruption due tousing the second source of timing to adjust the clock rates.

System

FIG. 8 illustrates an example of a special purpose computer systems 800configured with components of headend 102 and/or remote physical device104 according to one embodiment. Computer system 800 includes a bus 802,network interface 804, a computer processor 806, a memory 808, a storagedevice 810, and a display 812.

Bus 802 may be a communication mechanism for communicating information.Computer processor 806 may execute computer programs stored in memory808 or storage device 808. Any suitable programming language can be usedto implement the routines of particular embodiments including C, C++,Java, assembly language, etc. Different programming techniques can beemployed such as procedural or object oriented. The routines can executeon a single computer system 800 or multiple computer systems 800.Further, multiple computer processors 806 may be used.

Memory 808 may store instructions, such as source code or binary code,for performing the techniques described above. Memory 808 may also beused for storing variables or other intermediate information duringexecution of instructions to be executed by processor 806. Examples ofmemory 808 include random access memory (RAM), read only memory (ROM),or both.

Storage device 810 may also store instructions, such as source code orbinary code, for performing the techniques described above. Storagedevice 810 may additionally store data used and manipulated by computerprocessor 806. For example, storage device 810 may be a database that isaccessed by computer system 800. Other examples of storage device 810include random access memory (RAM), read only memory (ROM), a harddrive, a magnetic disk, an optical disk, a CD-ROM, a DVD, a flashmemory, a USB memory card, or any other medium from which a computer canread.

Memory 808 or storage device 810 may be an example of a non-transitorycomputer-readable storage medium for use by or in connection withcomputer system 800. The non-transitory computer-readable storage mediumcontains instructions for controlling a computer system 800 to beconfigured to perform functions described by particular embodiments. Theinstructions, when executed by one or more computer processors 806, maybe configured to perform that which is described in particularembodiments.

Computer system 800 includes a display 812 for displaying information toa computer user. Display 812 may display a user interface used by a userto interact with computer system 800.

Computer system 800 also includes a network interface 804 to providedata communication connection over a network, such as a local areanetwork (LAN) or wide area network (WAN). Wireless networks may also beused. In any such implementation, network interface 804 sends andreceives electrical, electromagnetic, or optical signals that carrydigital data streams representing various types of information.

Computer system 800 can send and receive information through networkinterface 804 across a network 814, which may be an Intranet or theInternet. Computer system 800 may interact with other computer systems800 through network 814. In some examples, client-server communicationsoccur through network 814. Also, implementations of particularembodiments may be distributed across computer systems 800 throughnetwork 814.

Particular embodiments may be implemented in a non-transitorycomputer-readable storage medium for use by or in connection with theinstruction execution system, apparatus, system, or machine. Thecomputer-readable storage medium contains instructions for controlling acomputer system to perform a method described by particular embodiments.The computer system may include one or more computing devices. Theinstructions, when executed by one or more computer processors, may beconfigured to perform that which is described in particular embodiments.

As used in the description herein and throughout the claims that follow,“a”, “an”, and “the” includes plural references unless the contextclearly dictates otherwise. Also, as used in the description herein andthroughout the claims that follow, the meaning of “in” includes “in” and“on” unless the context clearly dictates otherwise.

The above description illustrates various embodiments along withexamples of how aspects of particular embodiments may be implemented.The above examples and embodiments should not be deemed to be the onlyembodiments, and are presented to illustrate the flexibility andadvantages of particular embodiments as defined by the following claims.Based on the above disclosure and the following claims, otherarrangements, embodiments, implementations and equivalents may beemployed without departing from the scope hereof as defined by theclaims.

What is claimed is: 1-20. (canceled)
 21. An apparatus communicativelycoupled remotely to a remote device and to a source of timinginformation that the first communication device and the remote deviceuse to maintain synchronization, the apparatus comprising: a firstclock; a processor capable of, upon detecting of a loss of a connectionto the source of timing information: selectively receiving second timinginformation from the remote device, the second source of timinginformation independent of the first source of timing information andalso being transmitted to another remote device; using the second sourceof timing information to determine a first timestamp; determining asecond timestamp from the first clock; and using the first timestamp andthe second timestamp to adjust a rate of the first clock; wherein thefirst clock is used to transmit the second source of timing informationto the another remote device.
 22. The apparatus of claim 21 where thesecond source of timing information comprises a map that determines whenthe another remote device transmits upstream to the remote device. 23.The apparatus of claim 22 where the map defines a plurality of times forwhen the another remote device is assigned to transmit upstream to theremote device.
 24. The apparatus of claim 21 where the second source oftiming information includes a value that defines a slot in which theanother remote device transmits upstream to the remote device.
 25. Theapparatus of claim 24, where the value is a minislot value that definesa time for the slot in which the another remote device transmitsupstream to the remote device.
 26. The apparatus of claim 21 where theprocessing device is configured to compare the first timestamp and thesecond timestamp and adjust the rate of the first clock based on thecomparison.
 27. The apparatus of claim 21 where the processing device isconfigured to calculate a difference between the first timestamp and thesecond timestamp and use the difference to adjust the rate of the firstclock.
 28. The apparatus of claim 27 where the processor adjusts thefirst clock based on the difference compared to an expected difference.29. The apparatus of claim 28, where the expected difference comprises atime within which the apparatus is expected to receive and process thesecond source of timing information and send a communication upstream tothe remote device at a time corresponding to the first timestamp. 30.The apparatus of claim 28 where the processor learns the expecteddifference before the connection to the first source of timinginformation is detected as lost.
 31. The apparatus of claim 27 where theprocessor adjusts the rate of the first clock based on receiving aplurality of differences calculated from receiving multiple instances ofthe second source of timing information from the remote device.
 32. Theapparatus of claim 21 where the first source of timing information is aseparate timing server and is used to lock a rate of the first clock toa second clock.
 33. The apparatus of claim 32 where the adjustment ofthe first clock does not lock the first clock to the same rate as thesecond clock.
 34. The method of claim 21 where one or more separatetiming servers are used to provide the first source of timinginformation to the apparatus and the remote device.